How to change camera movement speed on Path during animation
Make Sure you create Keyframes for the Offset value in the Follow Path Constraint.




Insert Keyframes on the [Follow Path Constraint].  Use [Fixed Position] TRUE.  The image belows shows two keyframes in the timeline and the dopesheet.





By using keyframes to directly and easily change position, you will change the speed, usefully and indirectly.  The object can also stop moving by using the same value of keyframe at two different frames. You can also reverse direction. You will be specifying the positions (0 = start, .5 = mid, 1.0 end) along the curve.  There is no direct settings for distance per frame and this is not a large problem under many scenarios.
There are also many tutorials at video sites that will visually explain this.  Consider searching Blender Tutorial Follow Path Constraint.